U.S. patent number 7,742,566 [Application Number 11/952,498] was granted by the patent office on 2010-06-22 for multi-energy imaging system and method using optic devices.
This patent grant is currently assigned to General Electric Company. Invention is credited to Peter Michael Edic, Forrest Frank Hopkins, Susanne Madeline Lee.
United States Patent |
7,742,566 |
Hopkins , et al. |
June 22, 2010 |
**Please see images for:
( Certificate of Correction ) ** |
Multi-energy imaging system and method using optic devices
Abstract
A multi-energy imaging system and method for selectively
generating high-energy X-rays and low-energy X-ray beams are
described. A pair of optic devices are used, one optic device being
formed to emit high X-ray energies and the other optic device being
formed to emit low X-ray energies. A selective filtering mechanism
is used to filter the high X-ray energies from the low X-ray
energies. The optic devices have at least a first solid phase layer
having a first index of refraction with a first photon transmission
property and a second solid phase layer having a second index of
refraction with a second photon transmission property. The first
and second layers are conformal to each other.
Inventors: |
Hopkins; Forrest Frank (Cohoes,
NY), Lee; Susanne Madeline (Cohoes, NY), Edic; Peter
Michael (Albany, NY) |
Assignee: |
General Electric Company
(Niskayuna, NY)
|
Family
ID: |
40512437 |
Appl.
No.: |
11/952,498 |
Filed: |
December 7, 2007 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20090147922 A1 |
Jun 11, 2009 |
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Current U.S.
Class: |
378/84;
250/505.1; 378/158; 378/85; 378/159; 378/147; 378/156 |
Current CPC
Class: |
G21K
1/10 (20130101); H01J 35/16 (20130101); B82Y
10/00 (20130101); G21K 1/062 (20130101); H01J
2235/183 (20130101); G21K 2201/061 (20130101); G21K
2201/064 (20130101); G21K 2201/067 (20130101) |
Current International
Class: |
G21K
1/06 (20060101); G21K 3/00 (20060101) |
Field of
Search: |
;378/84,85,145,147,149,156,157,158,159 ;250/505.1 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Tournear et al., "Gamma-Ray Channeling in Layered Structures",
IEEE, pp. 4282-4285, 2004. cited by other .
PCT International Search Report dated Jul. 7, 2009. cited by
other.
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Primary Examiner: Ho; Allen C.
Attorney, Agent or Firm: Asmus; Scott J.
Claims
What is claimed as new and desired to be protected by Letters
Patent of the United States is:
1. An optic assembly, comprising: an optic device for transmitting
a desired range of X-ray energies though total internal reflection,
comprising at least three conformal solid phase layers, wherein
interfaces between said solid phase layers are gapless and wherein
said at least three conformal solid phase layers include at least
one X-ray redirection region; and a filtering mechanism for
filtering out certain energies from a beam transmitted by said
optic device, wherein said filtering mechanism is at least one of a
filtering apparatus external to said optic device and a filtering
apparatus integral to said optic device.
2. The assembly of claim 1, wherein said filter apparatus comprises
at least one roughened interface surface.
3. The assembly of claim 1, wherein said filter apparatus comprises
a filter wheel.
4. The assembly of claim 1, wherein said filter apparatus comprises
a dopant.
5. The assembly of claim 1, wherein said filter apparatus comprises
a vapor deposited or chemically plated material on an input or an
output face of said optic device.
6. The assembly of claim 1, wherein said filter apparatus comprises
a choice of different materials in said optic device that determine
the critical angle for total internal reflection, wherein the
critical angle determines the highest X-ray energies transmitted by
said optic device.
7. An array of optic devices, comprising: a first optic portion for
transmitting first optic X-ray energies though total internal
reflection; and a second optic portion for transmitting second
optic X-ray energies, said second optic X-ray energies less than or
equal to said first optic X-ray energies, wherein either or both of
said first and second optic portions comprises at least three
conformal solid phase layers, wherein interfaces between said solid
phase layers are gapless and wherein said at least three conformal
solid phase layers include at least one X-ray redirection region,
and wherein at least two of said layers have different indices of
refraction.
8. The array of claim 7, wherein said first and second optic
portions comprise opposing halves of a single optic device.
9. The array of claim 7, wherein said first optic portion is formed
of materials that pass X-ray energies greater than said second
optic X-ray energies.
10. A method for forming a limited energy spectrum image by: taking
an image with x-ray energies transmitted though an optic device
using total internal reflection; taking a second image with fewer
x-ray energies that have been transmitted by the optic device
utilizing a filtering mechanism, wherein said filtering mechanism
is at least one of a filtering apparatus external to said optic
device and a filtering apparatus integral to said optic device, and
subtracting the second image from the first image.
11. A multi-energy imaging system, comprising: a source of
electrons; a target for forming X-rays upon being struck by
electrons from said source of electrons; a vacuum chamber housing
the target; a window though which the X-rays may exit the vacuum
chamber; at least one optic device configured to transmit a desired
range of X-ray energies, said at least one optic device comprises:
a first optic portion for redirecting first optic X-rays though
total internal reflection; and a second optic portion for
redirecting second optic X-rays, said second optic X-rays being at
a lower energy level than said first optic X-rays; and wherein said
at least one optic device comprises at least three conformal solid
phase layers, wherein interfaces between said solid phase layers
are gapless.
12. The multi-energy imaging system of claim 11, wherein: said
first optic portion is configured for producing a first optic
energy spectrum; and said second optic portion is configured for
producing a second optic energy spectrum, wherein said first optic
energy spectrum contains energies less than or equal to the
energies in the said second optic energy spectrum.
13. The multi-energy imaging system of claim 11, comprising a
filtering mechanism for filtering out certain energies from a beam
transmitted by said at least one optic device, wherein said
filtering mechanism is at least one of a filtering apparatus
external to said optic device and a filtering apparatus integral to
said optic device.
14. The multi-energy imaging system of claim 13, wherein said
filter apparatus comprises at least one roughened interface
surface.
15. The multi-energy imaging system of claim 11, wherein said at
least one optic device comprises at least three conformal solid
phase layers, wherein interfaces between said solid phase layers
are gapless and wherein said at least three conformal solid phase
layers include at least one X-ray redirection region.
16. The multi-energy imaging system of claim 15, wherein said
filter apparatus comprises a filter wheel.
17. The multi-energy imaging system of claim 15, wherein said
filter apparatus comprises a dopant.
18. The multi-energy imaging system of claim 15, wherein said
filter apparatus comprises a vapor deposited or chemically plated
material on an input or an output face of said optic device.
19. The multi-energy imaging system of claim 15, wherein said
filter apparatus comprises a choice of different materials in said
optic device that determine the critical angle for total internal
reflection, wherein the critical angle determines the highest X-ray
energies transmitted by said optic device.
20. The multi-energy imaging system of claim 11, wherein said at
least one optic device comprises a pair of optic devices.
21. A method for manufacturing a multi-energy imaging system for
filtering different energy level X-rays through total internal
reflection in an imaging system, comprising: providing a target
configured to form X-rays upon being struck with electron beams;
and providing at least one optic device in optical communication
with the target, the at least one optic device being formed to
transmit one level of X-ray energies, wherein said at least one
optic device comprises at least three conformal solid phase layers,
wherein interfaces between said solid phase layers are gapless and
wherein said at least three conformal solid phase layers include at
least one X-ray redirection region.
22. The method of claim 21, comprising providing a filter for
selectively separating the different energy level X-rays.
23. The method of claim 22, wherein said filter comprises at least
one roughened interface surface.
24. The method of claim 22, wherein said filter apparatus comprises
a filter wheel.
25. The method of claim 22, wherein said filter apparatus comprises
a dopant.
26. The method of claim 22, wherein said filter apparatus comprises
a vapor deposited or chemically plated material on an input or an
output face of said optic device.
27. The method of claim 22, wherein said filter apparatus comprises
a choice of different materials in said optic device that determine
the critical angle for total internal reflection, wherein the
critical angle determines the highest X-ray energies transmitted by
said optic device.
Description
BACKGROUND
The invention relates generally to optics, and more particularly to
multilayer optic devices and methods for making the same.
Numerous applications exist that require a focused beam of
electromagnetic radiation. For example, energy dispersive X-ray
diffraction (EDXRD) may be used to inspect checked airline baggage
for the detection of explosive threats or other contraband. Such
EDXRD systems may suffer from high false positives due to weak
diffracted X-ray signals. The weakness of the X-ray signals may
stem from a variety of origins. First, the polychromatic X-ray
spectrum used in EDXRD is produced by the Bremsstrahlung part of
the source spectrum, which is inherently low in intensity. Second,
X-ray source collimation may eliminate more than 99.99 percent of
the source X-rays incident on the baggage volume under analysis.
Third, some of the materials being searched for, e.g., explosives,
may not diffract strongly as they are amorphous. Fourth, the
diffracting volume may be small. The last two limitations arise
from the type of threat materials being searched for in baggage,
making all but the second limitation unavoidable. Although
discussed in the context of explosives detection, the limitations
described above are equally applicable to medical situations.
At lower X-ray energies, such as 80 keV and below, increasing the
polychromatic X-ray flux density at the material being inspected
has been addressed by coupling hollow glass polycapillary optics to
low powered, sealed tube (stationary anode) X-ray sources. An
example of hollow glass polycapillary optics may be found in, for
example, U.S. Pat. No. 5,192,869. The glass is the low index of
refraction material, and air filling the hollow portions is the
high index of refraction material. These types of optics typically
do not provide much gain at energy levels above 80 keV, since the
difference in the indices of refraction between air and glass
becomes increasingly small as energy levels approach and surpass 80
keV.
Further, such optics use a concept of total internal reflection to
reflect X-rays entering the hollow glass capillaries at appropriate
angles back into the hollow capillaries, thereby channeling a solid
angle of the source X-rays into collimated or focused beams at the
output of the optic. As used herein, the term "collimate" refers to
altering the divergence of beams of electromagnetic (EM) radiation
from the intrinsically divergent EM beams. Only about five percent
of an EM source's solid angle typically is captured by the input of
such known optics.
In addition, the use of air in known optics as one of the materials
prevents such optics from being placed within a vacuum. Thus, known
optics are limited in their potential uses.
The shaping of an X-ray spectrum to optimize it for particular
applications is a common procedure. The change in the spectral
shape, for example, reducing either the relative proportion of
low-energy X-rays or the relative proportion of high-energy X-rays,
can in some cases provide for optimum imaging of a sample. One
common artifact in radiographic and tomographic imaging arises from
the fact that the lower energy X-rays in a typical Bremsstrahlung
(polychromatic) spectrum are attenuated preferentially as the beam
penetrates material. This effect, which leads to an increase in the
mean energy of the beam as it penetrates the sample, introduces a
biasing in the relationship between the strength of the transmitted
beam and the amount of material penetrated. This biasing manifests
as artifacts in any images reconstructed from the attenuation data,
such as those attributed to beam hardening in computed tomography.
Utilizing an X-ray beam that has a reduced spread of energies can
mitigate some of these artifacts. Particularly where beam
intensity, with respect to the intensity in that same range of the
original spectrum, has been held constant or augmented by the use
of the optic, the use of a limited range of energies can provide a
desired degree of attenuation for a particular application and can
produce an optimum image in terms of spatial resolution and
contrast sensitivity. The shaping of a spectrum from a
polychromatic energy distribution to a more monochromatic
distribution can enable such improvements in X-ray image sets.
Spectral imaging also includes a single energy distribution as well
as multi-energy distributions. Multi-energy X-ray imaging,
sometimes referred to as dual-energy imaging or energy
discrimination imaging, has been shown to furnish information on
specific material compositions in scanned objects for security,
industrial, and medical applications. Such energy discrimination
imaging can be achieved in several ways, including the use of two
or more different X-ray spectra, which is often the most feasible
approach. A challenge lies in the sequential nature of such an
examination, where image data are generated, for example, first
with one spectrum and then with another spectrum. In one technique,
an object of interest is scanned twice. A first complete projection
data set is produced in the first scan for one energy and then a
second complete projection data set is produced in the second scan
for the second energy. For many applications where high throughput
is critical, sample composition is dynamic, and/or sample
positioning may preclude repetitive scanning, the logistics of
physically scanning an object twice may be unacceptable.
Conventional multi-energy X-ray imaging applications have used
source filtration and/or high voltage modulation for rapidly
altering the spectral characteristics on a time scale comparable to
the view-by-view sampling time in a typical CT scan. Such
filtration consists of rapidly and sequentially inserting filters
of appropriate composition to preferentially attenuate relatively
low X-ray energies. Such methodologies are limited in the degree to
which attenuation can produce cleanly separated energy intervals,
severely restricting the sensitivity of this approach for analyzing
different materials. High voltage modulation to produce different
spectral characteristics also has been implemented in some cases
with limited success. There is a challenge in both approaches to
mitigate registration differences in the image reconstruction
projections that result from sample movement between data sets
acquired at different energies, as well as a slight misalignment of
the X-ray paths that traverse the object, as is incurred with
modulating the X-ray beam on a sub-view basis.
BRIEF DESCRIPTION
The invention includes embodiments that relate to an optic assembly
that includes an optic device and a filtering mechanism. The optic
device transmits a desired range of X-ray energies through at least
one of total internal reflection, diffraction, and refraction. The
optic device includes at least three conformal solid phase layers,
wherein interfaces between the solid phase layers are gapless and
wherein the at least three conformal solid phase layers include at
least one X-ray redirection region. The filtering mechanism filters
out certain energies from a beam transmitted by the optic device.
The filtering mechanism is at least one of a filtering apparatus
external to the optic device and a filtering apparatus integral to
the optic device.
The invention includes embodiments that relate to an array of optic
devices that includes a first optic portion for transmitting high
X-ray energies or high and low X-ray energies and a second optic
portion for transmitting low X-ray energies.
The invention includes embodiments that relate to a method for
forming a high-energy spectrum image by subtracting a low-energy
spectrum from a high-and a low-energy spectrum image. The method
includes transmitting high and low X-ray energies through an optic
device using at least one of total internal reflection,
diffraction, and refraction. The method also includes filtering out
certain energies from a beam transmitted by the optic device to
generate the high-energy spectrum image utilizing a filtering
mechanism, wherein the filtering mechanism is at least one of a
filtering apparatus external to the optic device and a filtering
apparatus integral to the optic device.
The invention includes embodiments that relate to a multi-energy
imaging system that includes a source of electrons, a target for
forming X-rays upon being struck by electrons from the source of
electrons, a vacuum chamber housing the target, and a window
through which the X-rays may exit the vacuum chamber. The system
also includes at least one optic device configured to transmit a
desired range of X-ray energies.
The invention includes embodiments that relate to a method for
manufacturing a multi-energy imaging system for filtering
low-energy X-rays from high-energy X-rays in an imaging system. The
method includes providing a target configured to form X-rays upon
being struck with electron beams and providing at least one optic
device in optical communication with the target. The at least one
optic device is formed to transmit high X-ray energies or to
transmit low X-ray energies.
These and other advantages and features will be more readily
understood from the following detailed description of preferred
embodiments of the invention that is provided in connection with
the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view illustrating the phenomenon of total
internal reflection.
FIG. 2 is a top schematic view of an optic device constructed in
accordance with an embodiment of the invention.
FIG. 3 is a cross-sectional view of the optic device of FIG. 2
taken along line III-III.
FIG. 4 is an end view of the optic device of FIG. 2.
FIG. 5 is a perspective view of the optic device of FIG. 2.
FIG. 6 is a perspective view of an optic device constructed in
accordance with an embodiment of the invention.
FIG. 7 is a perspective view of an optic device constructed in
accordance with an embodiment of the invention.
FIG. 8 is a perspective view of an optic device constructed in
accordance with an embodiment of the invention.
FIG. 9 is a perspective view of an optic device constructed in
accordance with an embodiment of the invention.
FIG. 10 is a perspective view of an optic device constructed in
accordance with an embodiment of the invention.
FIG. 11 is a perspective view of an optic device constructed in
accordance with an embodiment of the invention.
FIG. 12 is a perspective view of an optic device constructed in
accordance with an embodiment of the invention.
FIG. 13 is a schematic view of a deposition assembly constructed in
accordance with an embodiment of the invention.
FIG. 14 is a schematic view of a deposition assembly constructed in
accordance with an embodiment of the invention.
FIG. 15 illustrates process steps for fabricating an optic device
in accordance with an embodiment of the invention.
FIG. 16 is a block diagram of a conventional computed tomography
system.
FIG. 17 is a cross-sectional view of an X-ray tube head using an
optic in accordance with an embodiment of the invention.
FIG. 18 is a schematic view of a pair of optic devices for use with
a target in a dual energy scanning system in accordance with an
embodiment of the invention.
FIG. 19 is a schematic view of a filter wheel for use with the
optic devices of FIG. 18.
FIG. 20 is a schematic view of a spliced optic device for use with
a target in a dual energy scanning system in accordance with an
embodiment of the invention.
DETAILED DESCRIPTION
Embodiments of the invention described herein primarily utilize the
phenomenon of total internal reflection. Referring to FIG. 1, when
an angle of incidence is less than a critical angle .theta..sub.c,
total internal reflection occurs. The critical angle .theta..sub.c
for total internal reflection depends on, among other factors, the
selection of materials, the difference in the relative indices of
refraction between the materials, the material photon absorption
properties, and the energy of the incident photons. By appropriate
material selection in the multilayer optic described herein, the
critical angle .theta..sub.c can be increased several times over an
air-glass critical angle, allowing many more photons to satisfy the
condition for total internal reflection. This will allow a greater
photon transmission through a multilayer optic than is possible
with, for example, polycapillary optics.
Referring now to FIGS. 2-5, there is shown a multilayer optic 10
including an input face 12 and an output face 14. By "multilayer"
is meant a structure that has a plurality of layers with each layer
having a single composition. As shown more particularly in FIGS. 3
and 4, the multilayer optic 10 includes multiple layers of
material, each having a different index of refraction. For example,
there are layers 16, 20, and 24 surrounding a core 50. Layer 15,
formed of a lower index of refraction material, is positioned
radially exterior to and contiguous with the core 50. The core 50
may be formed of a higher index of refraction material such as
beryllium, lithium hydride, magnesium, or any other suitable
elements or compounds having similarly higher refractive indices
and high X-ray transmission properties. The core 50 may be less
than a micrometer to greater than one centimeter in diameter. Layer
20 is positioned radially exterior to layer 16 and radially
interior to layer 24.
In one embodiment, the layers making up the multilayer optic 10 may
be formed of materials that have varying indices of refraction. For
example, layers 15, 19, 23, and 27 may be formed of materials that
have a lower index of refraction and a high X-ray absorption. For
example, appropriate materials may be chosen from osmium, platinum,
gold, or any other suitable elements or compounds having similarly
lower refractive indices and high X-ray absorption properties.
Further, the core 50 and layers 16, 20, and 24 may be formed of
materials having a higher index of refraction and a high X-ray
transmission. For example, appropriate materials may be chosen from
beryllium, lithium hydride, magnesium, or any other suitable
elements or compounds having similarly higher refractive indices
and high X-ray transmission properties. The diameter of the core 50
is computed by considering the location of the X-ray radiation
source focal point relative to the input face of the optic and the
required critical angle for total internal reflection between the
higher index of refraction of the core 50 and the lower index of
refraction of the layer 15.
By using alternating lower and higher index of refraction materials
with concurrent high and low X-ray absorption properties,
respectively, in contiguous layers, the multilayer optic 10 can
utilize the principle of total internal reflection of
electromagnetic radiation. Specifically, diverging electromagnetic
radiation beams 38, 40, and 42 stemming from an electromagnetic
radiation source 34 enter the input face 12 and are redirected by
the principle of total internal reflection into quasi-parallel
beams of photons 44 exiting the output face 14.
Multilayer optics in accordance with embodiments of the invention,
such as optic 10, can collect a large solid angle of an X-ray
source 34 and redirect photons containing polychromatic energies
into quasi-parallel photon beams. "Quasi-parallel" means that
diverging beams of photons, such as X-rays, have been collected and
focused into beams of electromagnetic radiation or X rays to exit
the output face 14 at or below the critical angle .theta..sub.c.
This divergence causes the intrinsic source X-ray beam to be larger
than the output face 14 of the optic 10 and larger than the
parallel beam of X rays produced by the optic. Alternatively,
multilayer optics in accordance with embodiments of the invention
may be configured to produce slightly focused, highly focused,
slightly diverging, or highly diverging beams. By "slightly
focused" is meant that the beam size at the point of interest
(i.e., where the diameter of the beam is of concern) is
approximately the same as the beam at the output face 14 of the
optic 10. By "highly focused" is meant that the beam size at the
point of interest is smaller than the beam at the output face 14 of
the optic 10. By "slightly diverging" is meant that the beam size
is larger than a quasi-parallel beam but smaller than the intrinsic
source beam. By "highly diverging" is meant that the beam is the
same size or larger than the intrinsic source beam. The phrase
"intrinsic source beam" is meant to represent an X-ray beam emitted
from the source housing with no optic in the beam.
The composition of materials making up the multilayer optic 10, the
macroscopic geometry of the multilayer optic 10, the thickness of
the multilayer optic 10, and the number of individual layers
determine the angular acceptance range of the multilayer optic 10.
The angular acceptance range may be from about 0 steradians up to
about 2.pi. steradians of a solid angle of a photon source. For
ease of illustration, only a few layers have been illustrated with
reference to multilayer optic 10. However, it should be appreciated
that any number of layers, including into the hundreds, thousands,
or millions of layers, can be fabricated to utilize total internal
reflection to form the various types of photon beams listed
previously.
Another feature of the multilayer optic 10 is that the core 50 and
the layers 16, 20, 24 may have photon, or X-ray, redirection
regions. For example, layer 16 has a photon redirection region 17
stemming from a center of curvature; layer 20 has a photon
redirection region 21 stemming from a second center of curvature;
and, layer 24 has a photon redirection region 25 stemming from yet
another center of curvature. The photon redirection regions 17, 21,
25 are chosen to allow for the diverging electromagnetic radiation
beams 38, 40, and 42 to be made parallel or near parallel to beam
36, or conversely to allow for parallel or converging
electromagnetic radiation beams to be made diverging. The minimum
photon redirection region is determined by the minimum thickness
that would still enable a smooth surface, which is at least two
atomic layers, or about ten angstroms. The photon redirection
regions 17, 21, 25 each contain redirecting segments. The
redirecting segments are chosen such that they each have a constant
curvature. The curvature of each redirecting segment may be the
same as or different from the curvatures of other redirecting
segments. If each of the redirecting segments for a particular
photon redirection region is straight, then the radius of curvature
is infinite.
By curving the multilayers 16, 20, 24 at the input side of the
optic 10, the photons or electromagnetic radiation 38, 40, 42
entering the input face 12 can be redirected into quasi-parallel
pencil beams 44, thereby increasing the photon flux density at the
output face 14 over the photon flux density in the direct source
beam (X-ray beam without the optic) at the same distance from the
source 34. Depending upon the number of layers in the multilayer
optic, there may be a photon density gain for 100 keV photons of as
much as 5000 times in the output intensity of electromagnetic
radiation from the multilayer optic over the output of conventional
pinhole collimators. It should be appreciated that, alternatively,
the output face 14 may be formed closer to the input face 12, i.e.,
positioned prior to the region where the photons are redirected
into parallel rays, allowing the input electromagnetic radiation
beams 38, 40, 42 to remain somewhat diverging as they exit the
output face 14. It should further be appreciated that core 50 and
any number of the layers may have no arc of curvature, instead
having a cylindrical cross-sectional profile.
An important feature of this optic 10 is that the X-ray
transmitting layers, for example, layers 16, 20, 24, can be made
thin enough--on the order of a few nanometers--that the solid angle
of source photons collected by these layers are small enough to
accept almost all the X-rays entering the layers, i.e. the X-ray
trajectories satisfy the critical angle condition for total
internal reflection. This is unlike known optics, where the X-ray
transmitting regions are on the order of microns thick and a
significant number are absorbed at the reflecting interface because
the photon trajectories do not satisfy the critical angle condition
for total internal reflection. In addition, the X-ray absorption
layers are orders of magnitude thinner than in known optics making
the X-ray transmission of known optics orders of magnitude smaller
than the optics described in this application. Furthermore, the
overall optic length (from input face 12 to output face 14) is
short enough that photon losses are minimal.
Another feature of the multilayer optic 10 is that through
fabrication techniques that will be described in detail below, the
individual layers can be formed conformally on one another. The
conformation of the layers enables the multilayer optic 10 to be
utilized in a vacuum environment. Prior art optics utilize air as
the higher refractive index material. Such optics cannot be used in
vacuum environments. Further, the multilayer optic 10 can be
utilized in applications that operate at energy levels above 60
keV, such as, for example, X-ray diffraction, medical and
industrial CT imaging, medical and industrial X-ray, and cargo
inspection, to name a few. Some of these applications may operate
at energy levels as high as 450 keV.
Referring now to FIG. 6, there is shown a multilayer optic 110
including a plurality of layers 113a-113n, one on top of the other,
extending between an input face 112 and an output face 114 having a
polygonal profile. As illustrated, the middle layer of the
multilayer optic 110 is layer 113mid. Except for layer 113mid, all
of the layers include a photon redirection region positioned
between the input face 112 and the output face 114. It should be
appreciated, however, that layer 113 mid may include a photon
redirection region, or that other layers in addition to 113 mid may
lack a photon redirection region. The design shown allows diverging
electromagnetic radiation to be input into the input face 112,
redirected by the optic multilayers, and output from the output
face 114 into a reduced cone beam, such as, for example, a reduced
cone fan beam. Depending upon where the output face 114 is located
relative to the photon redirection regions, the fan beams may be
parallel or near parallel or may be somewhat divergent but still
focused relative to the input electromagnetic radiation.
Additionally, the conformal nature of the individual layers allows
for the multilayer optic 110 to be utilized in a vacuum
environment.
Referring to FIG. 7, there is shown a multilayer optic 210 that
includes an input face 212 and an output face 214. As with the
embodiment shown in FIG. 6, the multilayer optic 210 includes
individual layers sandwiching a mid-layer. The design shown allows
for a focused fan beam output. As with the previously described
embodiments, the conformal nature of the individual layers allows
the multilayer optic 210 to be used in a vacuum environment.
FIG. 8 illustrates a multilayer optic 310 having an input face 312
and an output face 314. The layers have been positioned over a cone
150, which serves as a blank or mold for the individual layers.
Through this design, the output beam exiting the output face 314 is
shaped into a curved output, which can be coupled to a singly
curved diffracting crystal (not shown) to enable the creation of a
cone beam of highly monochromatic radiation. Monochromatic
radiation is used in several different applications, including, for
example, X-ray diffraction. Highly monochromatic radiation is
radiation within a very narrow energy range approximately equal to
that produced by diffracting from a single crystal. The curved
diffracting crystal can be formed of any suitable material, such
as, for example, mica, silicon, germanium, or platinum and curved
so that the crystal conforms to the surface of, for example, a cone
or cylinder. The suitability of any material for use as the
diffracting crystal is dependent upon the diffraction intensity and
the lattice spacing of the material. It should be appreciated that
the multilayer optic 310 should be positioned between the source of
the electromagnetic radiation and the diffracting crystal to obtain
the maximum photon flux density in the diffracted beam.
Placing a filter at the input or the output faces of the optics in
FIGS. 5-7 will make the optics' output radiation
quasi-monochromatic. Quasi-monochromatic radiation is radiation
within a limited wavelength range that is greater than the highly
monochromatic range but less than the full Bremsstrahlung spectrum
from an X-ray source.
FIGS. 9-12 illustrate various other potential embodiments of
multilayer optics. FIGS. 9 and 10 illustrate multilayer optics that
have output faces in a photon redirection region, thereby allowing
such optics to emit highly diverging beams. FIGS. 11 and 12
illustrate multilayer optics whose output faces are dimensionally
smaller than their respective input faces, allowing such optics to
emit highly focused beams.
Referring now to FIG. 13, next will be described an apparatus for
use in forming a multilayer optic. Specifically, a multilayer optic
deposition assembly 400 is shown including a deposition chamber 402
and a movable shutter apparatus 410. The deposition chamber 402 may
be utilized in suitable deposition techniques, including, for
example, vapor deposition, or thermal spray deposition. Suitable
vapor deposition techniques include sputtering, ion implantation,
ion plating, laser deposition, evaporation, and jet vapor
deposition. Evaporation techniques may include thermal,
electron-beam, or any other suitable technique resulting in
appreciable deposition of material. Suitable thermal spray
deposition includes combustion, electric arc, and plasma spray. The
deposition chamber 402 includes an inputting apparatus 404 for
allowing ingress of deposition materials into the deposition
chamber 402. It should be appreciated that the inputting apparatus
404 may include numerous inlet nozzles, each being associated with
a specific deposition material. A blank 420 is positioned within
the deposition chamber 402. The blank 420 may be a core 50 or a
cone 150, described previously with regard to the embodiments
illustrated in FIGS. 4 and 8, or it may be a substrate serving as a
support mechanism for deposited layers. It should be appreciated
that the blank 420 can assume virtually any suitable geometric
configuration consistent with the desired beam profile. Examples of
the almost infinite number of suitable geometric configurations
include a circular wafer, a rectangular prism, a cone, a cylinder,
and an egg-shape, to name a few.
The shutter apparatus 410 enables the formation of a multilayer
optic wherein the individual layers have a photon redirection
region. Specifically, as a deposition material is input into the
deposition chamber 402 through the inputting apparatus 404, the
shutter apparatus 410 moves in a direction A relative to the blank
420. If the speed of the shutter apparatus 410 decreases as it
moves in the direction A, an increasing amount of deposition
material will contact the blank 420 in the direction A, thereby
enabling the formation of a multilayer optic with individual layers
having different thicknesses and having photon redirection regions.
Control of the movement and velocity of the shutter apparatus 410
may be accomplished electronically with a digital controlling
mechanism, such as a microcontroller, microprocessor, or computer.
Alternatively, control of the movement may be accomplished
manually, or mechanically, such as, pneumatically, hydraulically,
or otherwise.
By moving the shutter apparatus 410 along direction A as each
deposition material is input through the inputting apparatus 404
into the deposition chamber 402, the individual layers can be
deposited upon the blank 420, and a multilayer optic having
conformal individual layers, like the multilayer optic 110 shown in
FIG. 6, can be formed. In forming a multilayer optic like the
multilayer optic 110, the first layer to be laid down may be the
layer adjacent to mid-layer 113mid. Then, the subsequent layers
leading to and including layer 113a can be deposited. Then, the
partially formed multilayer optic can be turned over and the layers
leading to and including layer 113n can be deposited. Further,
assuming a constant rate of deposition material being injected into
the deposition chamber 402, if the shutter apparatus 410 is
programmed to begin with a first velocity, transition into a second
different velocity, and then transition back to the first velocity,
a multilayer optic like the multilayer optic 210 shown in FIG. 7
can be formed. It should be appreciated that the deposition rate of
the deposition material in the deposition chamber 402 may be
altered as well.
Instead of utilizing a shuttle apparatus 410, it is possible to
move at varying speeds the inputting apparatus 404 relative to the
blank 420. Further, it is possible to move at varying speeds the
blank 420 within the deposition chamber 402 relative to the
inputting apparatus 404.
Referring to FIG. 14, there is shown a multilayer optic deposition
assembly 500 that includes a deposition chamber 502 and the movable
shutter apparatus 410. The deposition chamber 502 includes the
inputting apparatus 404 that is the source of a vapor stream and a
pair of rotatable spindles 505. The spindles 505 are capable of
rotating in a direction B. Further, the spindles 505 each include a
pointed end that comes into contact with and holds the blank 420.
By rotating the spindles 505 in the same direction B the blank 420
can be rotated while deposition material is introduced into the
deposition chamber 502 though the inputting apparatus 404. Movement
of the shutter apparatus 410 in the direction A and rotation of the
blank 420 in the direction B will enable the formation of a
multilayer optic such as the multilayer optic 10 shown in FIG. 5.
Alternatively, the spindles 505 can remain in a non-rotating state
during a first set of deposition steps to form the layers adjacent
to layers 113mid to 113a in FIG. 6. Then, the spindles 505 can be
rotated to turn the partially formed multilayer optic one hundred
and eighty degrees around to allow for a second set of deposition
steps to form the layers leading to and including 113n to form the
multilayer optic 110.
Instead of utilizing a shutter apparatus 410, it is possible to
move the inputting apparatus 404 at varying speeds relative to the
blank 420 while the blank 420 is being rotated by the spindles 505.
Further, it is possible to move the spindles 505 and the blank 420
within the deposition chamber 402 at varying speeds relative to the
inputting apparatus 404.
Alternatively, while spinning the blank 420, the inputting
apparatus 404 may be kept stationary, with its vapor beams focused
to different heights along the blank 420. The resulting different
deposition rates will create the depth and laterally graded inputs
and outputs on the optic and will enable the formation of a
multilayer optic such as the multilayer optic 10 shown in FIG.
5.
FIG. 15 illustrates process steps for forming a multilayer optic in
accordance with an embodiment of the invention. At Step 600, a
material having a pre-determined index of refraction with a
pre-determined photon transmission coefficient is deposited. The
material is deposited on a blank or substrate, which may be a core,
a cone, or a polygonal support mechanism. It should be appreciated
that the blank or substrate may be incorporated within the
multilayer optic, such as the core 50, or may serve merely as a
mold, like cone 150. Then, at Step 605, another material having a
prescribed index of refraction with a photon transmission
coefficient is deposited onto the previous material in such a way
as to be conformal and have minimal void spaces. It should be
appreciated that each individual layer may be formed at thicknesses
in the range of one nanometer to thousands of nanometers. After
Step 605, the Steps 600 and 605 can be sequentially repeated to
prepare, for example, multiple pairs of layers, with each pair
having one layer having a first index of refraction with a first
photon transmission coefficient and a second layer having a second
index of refraction with a second photon transmission coefficient.
The deposition of the first and second materials may be
accomplished by any number of suitable processes, such as, for
example, vapor deposition, thermal spray deposition, or
electroplating. As noted previously, examples of suitable vapor
deposition techniques include sputtering, ion implantation, ion
plating, laser deposition (using a laser beam to vaporize a
material or materials to be deposited), evaporation, or jet vapor
deposition (using sound waves to vaporize a material or materials
to be deposited). Also, as noted previously, evaporation techniques
may be thermal, electron-beam or any other suitable technique that
will result in appreciable deposition of material. Examples of
suitable thermal spray deposition techniques include combustion,
electric arc, and plasma spray.
It should be appreciated that during the deposition process, the
partially formed multilayer optic may be rotated, oscillated, or
moved. It may be turned, and it may be subjected to a deposition
process whereby the deposition material is deposited at different
rates along the axis of the multilayer optic. In this way,
multilayer optics can be formed with various configurations and
profiles that will allow for a greater amount of electromagnetic
radiation to be collected from a source at the input of the optic,
parallel or near parallel beams of electromagnetic radiation to be
output from the multilayer optic, or beams of electromagnetic
radiation output from the multilayer optic to be shaped into pencil
beams, cone beams, fan beams, or curved in an arc, as an
example.
Multilayer optics in accordance with embodiments of the invention
may be used in various industrial applications. For example, a
multilayer optic formed to emit a quasi-parallel beam having a
circular cross-section may find utility in X-ray diffraction and
backscatter applications, such as non-destructive examination. A
multilayer optic formed to emit a slightly focused beam with a
circular cross-section may find utility in X-ray diffraction, X-ray
fluorescence, medical diagnostic or interventional treatments, and
non-destructive examination applications. Multilayer optics formed
to emit a highly focused beam having a circular cross-section may
find utility in X-ray fluorescence; medical diagnostic or
interventional treatments of, for example, small tumors; and,
non-destructive examination applications. Multilayer optics formed
to emit a slightly diverging beam having a circular cross-section
may find utility in computed tomography and X-ray diagnostic system
applications. Multilayer optics formed to emit a highly diverging
beam having a circular cross-section may find utility in
non-destructive examination applications requiring an increased
field-of-view, and in medical diagnostic or interventional imaging
and treatments requiring an increased field-of-view, such as the
imaging and treatment of large tumors.
One example of the utility of multilayer optics formed to emit a
variety of beam shapes is in medical interventional treatments,
such as treatment of tumors, where the optic shape is determined by
the tumor shape. Such multilayer optics would allow X rays to be
focused onto the tumor without irradiating nearby healthy tissue,
providing targeted treatment with a minimum of damage to
surrounding healthy tissue.
Multilayer optics formed to emit a fan beam in one plane that is
quasi-parallel, slightly focusing, highly focusing, slightly
diverging, or highly diverging in a direction transverse to the
plane may find utility in computed tomography, X-ray diagnostic
system, and non-destructive examination applications. The fan beam
may have a divergence the same as or greater than that of the
source. Alternatively, multilayer optics formed to emit a
quasi-parallel fan beam in one plane that is quasi-parallel,
slightly focused, highly focused, slightly diverging, or highly
diverging within the plane of the fan would produce a beam having a
rectangular cross-section that may find utility in computed
tomography, as well as non-destructive and medical examination
applications.
Multilayer optics formed to emit a fan beam in one plane that is
slightly or highly diverging in the direction transverse to the fan
beam plane may find utility in medical interventional applications,
such as close-up imaging to increase field-of-view. The divergence
in the direction transverse to the fan beam plane is equal to or
greater than the source divergence. Multilayer optics formed to
emit a fan beam in one plane that is quasi-parallel, slightly
focusing, highly focusing, slightly diverging, or highly diverging
perpendicular to the plane of the fan may find utility in computed
tomography, X-ray diagnostic system, and non-destructive
examination applications. The fan beam may have a divergence the
same as or greater than that of the source.
A multilayer optic coupled to a diffracting crystal may produce a
quasi-parallel monochromatic fan beam that may find utility,
provided the intensity is great enough, in medical imaging and
interventional treatments. Such monochromatic imaging would reduce
a patient's dose of X-rays while increasing the resolution, for
example, by reducing cone beam artifacts, and reducing
streaking/shading such as those incurred with beam hardening
effects.
FIG. 16 illustrates a conventional acquisition system 700 for use
in an object detection system, such as, for example, a computed
tomography (CT) scanner. The acquisition system 700 comprises a
scanner 702 formed of a support structure and internally containing
one or more stationary or rotationally distributed sources of X-ray
radiation (not shown in FIG. 16) and one or more stationary or
rotational digital detectors (not shown in FIG. 16), as described
in greater detail below. The scanner 702 is configured to receive a
table 704 or other support for an object to be scanned, such as,
for example, baggage or luggage or patients. The table 704 can be
moved through an aperture in the scanner to appropriately position
the subject in an imaging volume or plane that is scanned during
imaging sequences.
The system further includes a radiation source controller 706, a
table controller 708 and a data acquisition controller 710, which
may all function under the direction of a system controller 712.
The radiation source controller 706 regulates timing for discharges
of X-ray radiation which is directed from points around the scanner
702 toward a detector element on an opposite side thereof, as
discussed below. The radiation source controller 706 may trigger
one or more emitters in a distributed X-ray source at each instant
in time for creating multiple projections or frames of measured
data. In certain arrangements, for example, the X-ray radiation
source controller 706 may trigger emission of radiation in
sequences to collect adjacent or non-adjacent frames of measured
data around the scanner. Many such frames may be collected in an
examination sequence, and data acquisition controller 710, coupled
to detector elements as described below, receives signals from the
detector elements and processes the signals for storage and later
image reconstruction. In configurations described below in which
one or more sources are rotational, source controller 706 may also
direct rotation of a gantry on which the distributed source or
sources are mounted. Operation of the gantry also may be controlled
by the system controller 712 or a separate controller altogether.
Table controller 708, then, serves to appropriately position the
table and subject in a plane in which the radiation is emitted, or,
in the present context, or generally within a volume to be imaged.
The table may be displaced between imaging sequences or during
certain imaging sequences, depending upon the imaging protocol
employed. Moreover, in configurations described below in which one
or more detectors or detector segments are rotational, data
acquisition controller 710 may also direct rotation of a gantry on
which the detector or detectors are mounted.
System controller 712 generally regulates the operation of the
radiation source controller 706, the table controller 708 and the
data acquisition controller 710. The system controller 712 may thus
cause radiation source controller 706 to trigger emission of X-ray
radiation, as well as to coordinate such emissions during imaging
sequences defined by the system controller. The system controller
may also regulate movement of the table in coordination with such
emission to collect measurement data corresponding to volumes of
particular interest, or in various modes of imaging, such as
helical modes. Moreover, system controller 712 coordinates rotation
of a gantry on which the source(s), detector(s), or both are
mounted. The system controller 712 also receives data acquired by
data acquisition controller 710 and coordinates storage and
processing of the data.
It should be borne in mind that the controllers, and indeed various
circuitry described herein, may be defined by hardware circuitry,
firmware or software. Moreover, the controllers may be separate
pieces of hardware, as shown in FIG. 16, or integrated into one
piece of hardware. The particular protocols for imaging sequences,
for example, will generally be defined by code executed by the
system controllers. Moreover, initial processing, conditioning,
filtering, and other operations required on the measurement data
acquired by the scanner may be performed in one or more of the
components depicted in FIG. 16. For example, as described below,
detector elements will produce analog signals representative of
depletion of a charge in photodiodes positioned at locations
corresponding to pixels of the data acquisition detector. Such
analog signals are converted to digital signals by electronics
within the scanner, and are transmitted to data acquisition
controller 710. Partial processing may occur at this point, and the
signals are ultimately transmitted to the system controller for
further filtering and processing.
System controller 712 is also coupled to an operator interface 714
and to one or more memory devices 716. The operator interface may
be integral with the system controller, and will generally include
an operator workstation for initiating imaging sequences,
controlling such sequences, and manipulating measurement data
acquired during imaging sequences. The memory devices 716 may be
local to the imaging system, or may be partially or completely
remote from the system. Thus, imaging devices 716 may include
local, magnetic or optical memory, or local or remote repositories
for measured data for reconstruction. Moreover, the memory devices
may be configured to receive raw, partially processed or fully
processed measurement data for reconstruction. A monitor (not
shown) may also be connected to operator interface 714 to allow
viewing of scan data, reconstruction data, or otherwise processed
data.
System controller 712 or operator interface 714, or any remote
systems and workstations, may include software for image processing
and reconstruction. As will be appreciated by those skilled in the
art, such processing of CT measurement data may be performed by a
number of mathematical algorithms and techniques. For example,
conventional filtered back-projection techniques may be used to
process and reconstruct the data acquired by the imaging system.
Other techniques, and techniques used in conjunction with filtered
back-projection may also be employed. A remote interface 718 may be
included in the system for transmitting data from the imaging
system to such remote processing stations or memory devices.
FIG. 17 illustrates a portion of an acquisition subsystem 800 for
use in an object detection system, such as, for example, a computed
tomography (CT) scanner such as the scanner 702 of FIG. 16.
Specifically, FIG. 17 illustrates an X-ray tube head 840. A
multilayer optic 10 is incorporated within the system 800. The
alternating lower and higher index of refraction materials with
concurrent high and low X-ray absorption properties, respectively,
in contiguous layers, of a multilayer optic 10 utilize the
principle of total internal reflection of electromagnetic
radiation. In operation, a filament, such as, for example, a
tungsten filament within the cathode 742, is heated to emit an
electron beam 726, which is directed towards an anode 744 in which
resides the target 724. Thus, diverging X-ray beams emanating from
the target 724 enter the input face 12 and are redirected into
beams of photons 734 exiting the output face 14. The multilayer
optic 10 can be formed to output any desired beam. The multilayer
optic 10 can be positioned exterior or interior to the window 748.
The multilayer optic 10 is shown in both locations in FIG. 17 for
ease of illustration.
The multilayer optic 10 may be formed in such a way as to produce a
desired shaped beam of X-rays at energies of 20 keV and above
depending on the application, such as the beams of photons 734
shown in FIG. 17. The multilayer optic 10 for producing a limited
cone beam of X-rays can be formed as described above with reference
to FIGS. 2-5, with the exception being that the output face 14 is
formed closer to an input face 12, i.e., positioned prior to the
region where the photons are redirected into parallel rays. The
input face 12 may be flat or it may be curved to accept as much of
the source cone of X-rays from the target 724. This allows the
input X-ray beams to be shaped into a desired shaped beam 734.
A third-generation CT imaging system where the X-ray tube and
detector rotate about the imaging volume has been described herein;
however, the optic is equally applicable to alternate
configurations of third-generation technology, for example, with
industrial CT configurations where the X-ray source and detector
are held fixed and a stage rotates the object during data
acquisition.
Referring specifically to FIG. 18, there is shown a pair of optic
devices 10.sub.a, 10.sub.b. Each of these optic devices 10.sub.a,
10.sub.b is similar to the optic device 10 described with specific
reference to FIGS. 2-5. The difference between the optic devices
10.sub.a, 10.sub.b is that one is formed to pass higher X-ray
energies, while the other is formed to pass lower X-ray energies.
Shaping or filtering the source spectrum with the optic devices
10.sub.a, 10.sub.b offers the promise of rapidly producing spectral
shapes with sharp higher-energy cutoffs on a sub-view basis, which
improves material separation sensitivity and can eliminate most
registration issues. The capabilities for producing spectra with
desired spectral shapes and for producing them on a fast time scale
makes such optic devices particularly useful for multi-energy
imaging.
K-edge filters may be utilized to provide a sharp low-energy
cut-off for each optic 10.sub.a, 10.sub.b. One embodiment includes
vapor depositing the K-edge filter directly onto either end of the
optic 10.sub.a, 10.sub.b. Alternatively, the K-edge filter may be
formed as a separate foil aligned with the output or input of the
optic 10.sub.a, 10.sub.b. Then each optic 10.sub.a, 10.sub.b would
have its own different K-edge filter either integral to the optic
or separate from it.
The optic devices 10.sub.a, 10.sub.b, which as illustrated may be
in a stacked arrangement, are in optical communication with the
target 724 of the X-ray tube head 840 of the acquisition subsystem
800 (FIG. 17). Specifically, X-rays 733 formed by striking electron
beams at focal spots 725 on the target 724 are propagated from the
focal spots 725 toward the input faces 12 of the optic devices
10.sub.a, 10.sub.b. Alternatively, the focal spots 725 may each be
within separate individual target spots as opposed to the single
continuous target spot 724 or on separate non-contiguous targets.
The X-rays 733 are then focused by the optical devices 10.sub.a,
10.sub.b, as described above, and exit the output faces as
redirected X-rays 734. This geometry can be replicated to produce
an array of pairs of such spots, where a distributed array of x-ray
source spots is to be utilized.
To assist in separating high-and low-energy signals, a number of
options are possible. One such arrangement uses an optic device
with a separate K-edge filter to produce two signals whose energy
distributions are different from each other. This is done by taking
an image with one optic device and then retaking the image with
both the optic device and a K-edge filter to eliminate low energy
photons. Subtracting the two, appropriately normalized, signals
results in a signal with predominantly low energies, while the
signal produced by the combined optic device and K-edge filter
produces a signal with relatively higher energy photons.
Alternatively, two optic devices could be used in conjunction with
at least one K-edge filter. The two optic devices are made of
materials that result in x-ray redirection and transmission of two
photon energy ranges that may or may not overlap. Taking an image
with these two optic devices, repeating with the optic devices and
a K-edge filter that blocks the energies from the optic device that
transmits the lower energies, and subtracting the two,
appropriately normalized, images will result in an image derived
from only the low energies passed by the optic that transmitted the
lower energies. The lower energy spectrum image could be obtained
by subtracting this higher energy spectrum image, appropriately
normalized, from the image formed with photons from the combined
two optic devices and K-edge filter. To create a sharper low energy
cut-off in the lower energy image, a second K-edge filter could be
included that blocks the lowest energy photons from that optic.
Another option that can provide even greater energy separation
between the signals is to couple the optic devices to separate
targets at different accelerating potentials and taking sequential
images with x rays emitted by each accelerating potential/optic
combination.
To obtain the image sets produced by the different energy
distributions quickly and with the best possible statistical
definition, a filter wheel 775 (FIG. 19) may be used for sequential
filtering of the signals. For example, to generate a dual-energy
photon distribution, the filter wheel 775 could be made to include
portions 780 that are opaque to all photons and windows 782 that
are transparent to all the X rays. The windows 782 are only
illustrated partially around the filter wheel 775 for ease of
illustration purposes only. Alternatively, the windows 782 could be
covered by appropriate filtering material, such as that needed to
block the low-energy X rays from the higher-energy spectrum when
two optic devices are used for imaging. By rotating the filter
wheel 775, a portion 780 can be positioned between the optic device
10.sub.a and the detector, thereby blocking all the X rays
transmitted by the optic 10.sub.a from reaching the detector.
Simultaneously, a window 782 can be positioned between the other
optic device 10.sub.b and the detector, allowing all the X rays
transmitted by optic 10.sub.b to reach the detector. Then, the
filter wheel 775 is rotated to allow a window 782 to be positioned
between the optic device 10.sub.a and the detector 750, and a
portion 780 is positioned between the optic device 10.sub.b and the
detector 750, allowing X rays from only optic device 10.sub.a to be
received by the detector.
The optic devices 10.sub.a, 10.sub.b are fabricated such that each
filters out certain X-ray energy levels. Specifically, each optic
device 10.sub.a, 10.sub.b is fabricated from appropriate optic
materials that selectively redirect X-rays of a certain energy
level in the optic by predominantly total internal reflection. The
refractive indices of the materials used to fabricate the optic
devices determine the high-energy cutoff, establishing the emitted
spectrum high-energy endpoint.
As described previously, the optic devices include alternating high
and low refractive index materials deposited in layers, with
individual layers being in the nanometer thickness range. Each
layer at the input face 12 of an optic device 10 may be curved to a
different degree to capture a large source angle, such as,
forty-five degrees or more, and redirect the X-rays into a tightly
collimated fan-shaped beam. As described previously, combining some
sort of low energy x-ray filtering, such as K-edge filters, with
the higher-energy filtration of the optic devices 10.sub.a,
10.sub.b, different spectral shapes can be produced effectively.
The low energy filtering can take the form of stand-alone foils
that can be placed in the conventional post-optic emission
position, or between the source focal spot and the optic devices.
Alternatively, these low-energy filters can be vapor-deposited or
chemically plated onto either the input or output ends of each
optic. Another possibility is to incorporate the filter internally
to the optic devices as a dopant in the high refractive index
materials. Yet another method for filtering the lower energies from
an optic's transmission is to manufacture the optic with
selectively rough interfaces. The surface roughness will cause the
lowest energies to be scattered, while not affecting the reflection
of the higher energies. Additionally, a method for internally
filtering includes choosing different materials in the optic device
that determine the critical angle for total internal reflection,
wherein the critical angle determines the highest X-ray energies
transmitted by the optic device. The energy spectra produced by
these optics-filter alternatives can be shaped considerably
different from the source's inherent Bremsstrahlung emission, with
much sharper high and low-energy cutoffs.
Another topology for creating two different energy distribution
images is shown in FIG. 20 involving splicing together two halves
10', 10'' of two different optics to form a single optic device
10.sub.c. The creation of a single optic device 10.sub.c of this
type potentially reduces the distance between the centroids of the
two target spots 725 and so allows a smaller intrinsic beam spot
size to be used, more efficiently utilizing the target emission,
reducing target loading, and improving image registration.
Switching from one beam to the other could be achieved with a
filter wheel such as filter wheel 775.
The difference between the optic devices 10', 10'' is that one
transmits higher X-ray energies 734', while the other transmits
lower X-ray energies 734''. Shaping or filtering the source
spectrum with the optic devices 10', 10'' and appropriately
incorporated lower energy filtration of each optic spectrum offers
the promise of rapidly producing spectral shapes with sharp high
and low energy cutoffs on a sub-view basis, which improves material
separation sensitivity and can eliminate most registration issues.
The capabilities for producing spectra with desired spectral shapes
and on a fast time scale makes such optic devices particularly
useful for multi-energy imaging. In the acquisition subsystem 800
(FIG. 17), the optic device 10.sub.c, is in optical communication
with the target 724 of the X-ray tube head 840. Specifically, X
rays 733 formed by striking electron beams at focal spot 725 on the
target 724 are propagated from the focal spot 725 toward the input
faces 12', 12'', respectively, of the two halves 10', 10'' of the
optic device 10.sub.c. The X rays 733 are then focused by the two
halves 10', 10'', as described above with regard to the two optic
devices 10.sub.a, 10.sub.b (FIG. 18), and exit the output faces as
redirected higher and lower energy X-rays 734', 734''.
Yet another topology involves even closer spatial integration of
the lower and higher energy optic regions. A single optic device
can be made with multiple different sets of high and low refractive
index materials, allowing one optic to produce multiple energy
distributions simultaneously. Additionally, in an optic that
produces a dual-energy beam, for example, the portion of the optic
device used to produce the lower energy beam could have its
corresponding low energy filter (e.g., a K-edge filter)
incorporated internally as a dopant. Then the filter wheel would
simply contain alternating regions of open windows and the K-edge
filters for the optic layer that transmit the higher energy X rays.
As described previously, taking sequential images with first the
K-edge filter on the filter wheel in line with the optic, then with
the open window on the filter wheel in line with the optic, and
then subtracting the appropriately normalized images, dual energy
images can be created with spatially coincident spots having
essentially identical spot shapes (averaged over the nanometer
layer structure of the optic).
As noted previously, the K-edge filters may be incorporated as
dopants. As an example of how such doping can be accomplished,
suppose the K-edge of nickel is used to block the lower energy
photons and the high refractive index layers are made of all one
material, LiH. Then the LiH layer could be doped with Ni by
co-depositing the lithium hydride and nickel simultaneously, with
the minimum amount of nickel over the length of the optic needed to
block the desired lower energies. The required nickel concentration
can be calculated from the overall optic length, the X-ray
transmitting layer thickness, and the desired degree of lower
energy blocking. For an optic on the order of millimeters long, the
number of nickel atoms likely would be three or more orders of
magnitude lower than the number of lithium hydride molecules.
The dual-energy system described with reference to FIGS. 18-20 will
experience fewer registration issues in the image reconstruction
projections. By arranging a pair of optic devices 10.sub.a,
10.sub.b together with a rapidly actuated shutter, such as the
filter wheel 775, each optic device can be sequentially exposed,
thereby controlling which spectrum is emitted at a given time. Such
an arrangement can provide alternating spectral shapes on a
sub-millisecond timescale, which will mitigate registration
differences between the projection sets for the two energies.
In another topology, two optic devices may be used, one being
focused onto one X-ray focal spot and shapes the X-ray spectrum in
a desired manner, such as producing a lower energy spectrum. The
second optic is focused onto a second X-ray focal spot and shapes
the X-ray spectrum to produce a higher energy spectrum. Each optic
device redirects its output to probe the same volume of space. The
range of X-ray energies incident on the optic devices can be
rapidly changed by appropriate gridding of either X-ray focal spot,
or by redirecting a single beam to multiple X-ray focal spot
locations. For example, to probe the scanned object with a
low-energy spectrum, the opposite X-ray focal spot is gridded. If
the two X-ray focal spot locations utilize the same accelerating
potential, the optic devices can be made to filter the spectra as
desired. Alternatively, the X-ray focal spot locations may utilize
different accelerating potentials and a similar optic device to
affect the spectral differences. In yet another embodiment, both
accelerating potential and optic device characteristics can be
altered to shape the spectral characteristics of the X-ray beam.
Furthermore, beam filters can be used with the optic to further
shape the spectra as desired. Any of the approaches presented
herein may involve replication to provide multi-energy capability
for a distributed array of x-ray spots.
Such a multi energy system can be used in applications other than
computed tomography. For example, such a system can be used in
X-ray diffraction, as well as standard X-ray projection imaging. By
using quasi-monochromatic X-rays, the optics can maintain required
energy intensity, allowing the scanning speed to remain high.
While the invention has been described in detail in connection with
only a limited number of embodiments, it should be readily
understood that the invention is not limited to such disclosed
embodiments. Rather, the invention can be modified to incorporate
any number of variations, alterations, substitutions or equivalent
arrangements not heretofore described, but which are commensurate
with the spirit and scope of the invention. For example, while the
embodiments of the invention described with specific reference to
FIGS. 17-20 refer to an optic device 10, 10.sub.a, 10.sub.b,
10.sub.c, such description was for ease of description only and it
should be appreciated that any of the multilayer optic devices
described herein can be incorporated as appropriate. Furthermore,
while single energy and dual-energy techniques are discussed above,
the invention encompasses approaches with more than two energies.
Additionally, while various embodiments of the invention have been
described, it is to be understood that aspects of the invention may
include only some of the described embodiments. Accordingly, the
invention is not to be seen as limited by the foregoing
description, but is only limited by the scope of the appended
claims.
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